The present invention relates to an electrophotocopying machine and more particularly to an arrangement for removal of electrical charge from non-image-carrying portions of a photosensitive medium in an electrophotocopying machine.
In a conventional electrophotocopying machine, light reflected from an original document to be copied is directed by an exposure system onto the electrically charged surface of a photosensitive medium, the exposure system including lamp means for illuminating the original document, and the orginal document being protected from external light by an opaque cover or other means, which also permits removal of charges from peripheral portions of the photosensitive medium when a small original document it being copied. By this action, light coming from highly reflective portions of the original document which do not carry image elements causes the charge to be removed from the photosensitive medium, whereas the amount of light coming from image portions of the document, which are much less reflective, is insufficient to cause marked removal of the charge from the photosensitive medium, whereby there is defined on the photosensitive medium an electrostatic latent image constituted by charged portions corresponding to image portions of the original document and uncharged portions corresponding to non-image portions of the document. This exposure process may be effected as a flash exposure process in which the entire image of the original document is directed onto the surface of the photosensitive medium at once, or as a scanning process in which light from successive portions of the original document is directed onto successive portions of a moving photosensitive medium. Subsequent to exposure, development of the electrostatic latent image carried by the photosensitive medium is effected by applying, for example, resinous toner particles onto the image-carrying portion of the photosensitive medium, these particles adhering to charged portions of the photosensitive medium, but not to uncharged portions thereof, thus defining a pattern corresponding to the content of the original document. This toner pattern is subsequently transferred from the photosensitive medium onto a sheet of copy paper, for example, after which the pattern is fixed on the copy paper, which is then discharged to the exterior of the photocopying machine, and remnant toner particles are removed from the photosensitive medium by suitable cleaning means, while any remnant charge is also removed from the photosensitive medium in order to permit even charging or light exposure thereof prior to the next exposure process.
The exposure system and associated elements of a photocopying machine are designed to permit exposure of an area of the photosensitive medium of a set size corresponding to the size of a sheet of copy paper, or, in the case of a photocopying machine permitting production of photocopies at different magnifications, exposure of two or three areas of particular set sizes, since an exposure system permitting arbitrary selection of the size of the exposed area of the photosensitive drum, in order to match the size of an original document, would be too complex for practical purposes. If the size of an original document exactly matches the size of the exposed area of the photosensitive medium, in the exposure process, charge is removed from all portions of this area which are not required for defining image elements and so no toner particles adhere to these portions in the development process, and in the photocopy subsequently obtained, the corresponding portions of the copy paper are white. However, if the area of the projected image of the original document is smaller than the set area for formation of an image on the photosensitive medium, and if this entire area of the photosensitive medium is charged prior to the exposure process and the original document is not covered by an opaque cover, peripheral portions of this area of the photosensitive medium may remain charged, resulting in undesirable adhesion of toner particles to the photosensitive medium and production of an undesirable border around the information-carrying portion of a subsequently obtained photocopy. If the sheet of copy paper is smaller than the image-carrying portion of the photosensitive medium, a great amount of toner particles remains adhered to the photosensitive medium, thus imposing considerable burden on the above-noted cleaning means for removal of toner particles. This problem inevitably occurs in a photocopying machine for production of photocopies at different magnifications, as shown in FIG. 1.
In FIG. 1, there is shown a fragmentary top plan view of a known photosensitive medium or photoreceptor Ra, in which the two-dot chain line portions F1 indicate areas required for production of latent images for photocopies of equal magnification or a magnification of one, and the one-dot chain line portions F2 indicate areas required for production of latent images for photocopies of reduced magnification. Therefore, the area having width d1 between the portions F1 defines an intermediate non-latent image carrying portion in the case of equal magnification and that having width d2 between the portions F2 represents an intermediate non-latent image carrying portion for the reduced magnification. Similarly, area on each side of the photoreceptor Ra having width e1 between the side edge of the portion F1 and the corresponding side edge of the photoreceptor Ra defines side non-latent image carrying portions for equal magnification while the areas having width e2 between the side edge of the portion F2 and corresponding side edge of the photoreceptor Ra represents side non-latent image carrying portions for reduced magnification. The non-latent image carrying portions as described above are uniformly charged in a copying operation in a similar manner as in the image carrying portions, resulting in the inconveniences as described earlier. It should be noted here, however, that the intermediate non-latent image carrying portions having widths d1 and d2, and the area having width e3 equivalent to the difference between the areas having widths e1 and e2 at each side edge of the photoreceptor Ra are also used as image forming portions in many copying operations.
More specifically, the photosensitive medium portions on the sides of the area required for production of reduced magnification photocopies and intermediate portions between photosensitive medium areas for image-formation at any magnification are liable to remain charged after the exposure process. Such charge may be efficiently removed from the above-noted intermediate portions of the photosensitive medium by provision in the photocopying machine exposure system of a light transmitting element, such as described in U.S. Pat. No. 2,909,971, for direction of light onto such a portion in each exposure process. However, such a means is more particularly suited to use in a single-magnification photocopying machine, and does not resolve the problem of undesired charge remaining at the sides of the image-carrying areas of the photosensitive medium.
For overcoming the problems as described above, there has previously been proposed an arrangement, for example, as shown in U.S. Pat. No. 3,751,155, according to which charge erasure means is provided to selectively remove charge on the side portions of a photosensitive medium prior to the process of development of an electrostatic latent image carried by the photosensitive medium, this means comprising one or more selectively actuated light sources and means for directing light source or sources onto the side portions of the photosensitive medium.
However, the conventional means such as referred to above is intended simply to prevent undesired adhesion of toner particles to a photosensitive medium, and disregards the effects which are produced on characteristics of the photosensitive medium after prolonged use due to exposure of different portions thereof to different types of light. That is, referring again to FIG. 1, regardless of the size of the photocopy to be produced, the area F2 of the photosensitive medium employed for production of reduced-magnification photocopies is always exposed to the same type of light, whereas the side portions having width e3 at the side edges of the area F2 used for production of reduced-magnification photocopies and the area F1 used for production of photocopies of a magnification of one are sometimes exposed to light having a wavelength selected with reference to characteristics of the photosensitive medium to render the light sensitivity of the photosensitive medium generally equivalent to that of the human eye and sometimes to light which is simply intended to remove a charge from the photosensitive medium, with the result that characteristics of the portions F2 and of the portions having the width e3 of the photosensitive medium become different after a certain period of time, and when it is required to produce a photocopy of a magnification of one, there is a difference in image-quality in portions of the photocopy corresponding to the portions F2 and the portions with the width e3 of the photosensitive medium.
In more detail, for example, if the photosensitive medium is the so-called PVK.multidot.Se type constituted by a 0.3.mu. film of selenium and a 2.mu. film of polyvinyl carbasole successively deposited on a conductive base, and the light source employed for exposure purposes is a halogen lamp with a color temperature of 3000.degree. K, the spectroscopic sensitivity of the PVK.multidot.Se photosensitive medium is as indicated by the curve (i) of FIG. 2, while wavelength characteristics of light emitted by the halogen lamp are as indicated by the curve (ii) of FIG. 3, and, since the PVK.multidot.Se photosensitive medium and the halogen lamp are used together in the same photocopying machine, the relative sensitivity of the photosensitive medium is as indicated by the curve (iii) of FIG. 4, which is obtained by multiplying successive values of the curve (i) of FIG. 2 by corresponding values of the curve (ii) of FIG. 3. In order to bring the curve (iii) of FIG. 4 close to relative visual sensitivity, i.e., in order to make the sensitivity of the photosensitive medium effectively close to that of the human eye, as shown by the dotted lines of FIG. 4, the exposure system suitably includes a filter which prevents passage of radiation of short wavelength, for example, a filter manufactured by Sumitomo Chemical Industries, Ltd. Japan which filters out radiation of light having a nominal wavelength of 550 m.mu. or less. The resulting sensitivity curve when such a filter is included in the exposure system is the curve (iv). Conventionally, however, no such a filter is provided in the arrangement for erasure of charge from non-image carrying portions of the photosensitive medium, with the result that, although problems of excessive use of toner particles and of removal of excess toner particles may be avoided, different portions of the photosensitive medium are regularly exposed to different types of radiation, and the sensitivity curve for portions F2 (FIG. 1) of the photosensitive medium which are always exposed to the same type of radiation is as indicated by the curve (iv) of FIG. 4 and that of photosensitive medium portions having the width e3 (FIG. 1) being as indicated by the curve (iii), thus resulting in variation of image density in these different portions after use of the photocopying machine for a certain period of time.
This problem is not limited to a photocopying machine which permits production of photocopies at varying magnifications, but also occurs in photocopying machines in which photocopy paper of varying sizes is employed. The photosensitive medium portions having widths d1 and d2 (FIG. 1) between image-carrying portions also are of course exposed to different types of radiation at different times, but as the photosensitive medium is moved during use of the photocopying machine, a portion of the non-latent image carrying portions having widths d1 and d2 which is not exposed to image light from an original document in one exposure process is used as an image-forming portion of the photosensitive medium in a later exposure process, whereby the effects of different kinds of radiation are cancelled.
In conventional arrangements, it is sometimes also the practice to make the lamp means used for erasure of charge from non-image carrying portions different from the lamp means used for exposure purposes. When, for example, the non-image carrying portion charge erasure lamp is a tungsten lamp with a color temperature of 2,800.degree. K., the relevant characteristic curve is as indicated by a curve (v) of FIG. 3. In this case, the overall sensitivity curve of a PVK.multidot.Se photosensitive medium is slightly lower than the curve (iii) of FIG. 4, and spreads in the short wavelength regions, but it is still different from the curve (iv), and consequently the result is still that difference of density in images produced on different portions of the photosensitive medium occurs.